For a firsthand look at the failure of San Francisco's transit system, ride Muni to North Beach from downtown. Board a No. 30-Stockton bus at the Caltrain terminal at Fourth and King streets and begin a long and frustrating ride. After jogging over to Third Street, you poke along toward Market Street, sharing the street with convention-goers surging across the streets and cars heading downtown from the freeway.

You cross Market Street, after a long wait at the light, and then slog through the Union Square shopping district, dodging double-parked cars and waiting through stoplights, until speed picks up through the 3 1/2-block long Stockton Street tunnel. Emerging from the tunnel, you are mired in Chinatown traffic and, more than 20 minutes after you began, you arrive at Clay Street. From there, it gets slower, as the bus, now jammed with riders, competes with vehicle traffic and jaywalkers. You notice that pedestrians on the sidewalk are going faster than the bus. Half an hour after you boarded, you finally reach Washington Square.

You can blame the mayor, the Board of Supervisors, the surly bus driver or the Muni management for your slow ride, but the simple truth is that they are doing the best they can with what they have to work with. Fast, efficient transit is not possible if it is limited to buses running in mixed traffic on narrow streets in one of densest and busiest urban centers in the United States.

Throughout the world, cities have recognized that dense urban areas can be adequately served only by separate rail rapid transit. We have the BART and Muni Metro systems, which provide this level of service to some parts of the city, but the densest quarter, north of Market Street and east of Van Ness Avenue, has no subways. We passengers have to endure buses that crawl along at walking speed. The planned Central Subway, which is an extension of the new T-Muni-Metro line, is our chance to finally provide world-class service to this part of the city.

The Central Subway corridor is the fourth of four new rapid-transit corridors approved by voters in 1987 as part of the "four corridors plan." One, the Third Street lightrail line, has been built. Another two, Geary Boulevard and Van Ness Avenue (both of which are wide streets that can accommodate exclusive transit lanes), are slated to get Bus Rapid Transit. The Central Subway is the final link in this modern rapid transit system that is long overdue. A trip from Fourth and King streets to Chinatown that takes 22 minutes on a good day, longer if traffic is bad, will take seven minutes, every time, on a comfortable train.

The San Francisco Planning and Urban Research Association supports this project, but with a couple of stern warnings. As proposed, the subway fails to make Muni much more efficient. Because it goes only as far as Chinatown, it does not directly serve much of northeast part of the city, therefore requiring slow, inefficient duplicative bus service to be retained. The subway should be extended to North Beach, serving a large population and allowing easy transfers from other lines. This could be done quickly and relatively inexpensively because the plan already calls for tunneling all the way to Washington Square, where an additional station easily could be built. Subsequently, the line could be extended to Fisherman's Wharf and the Van Ness rapid transit line, and from there possibly to the Presidio.

Also, the Central Subway would not be able to handle the ridership that will result as the line is extended further and attracts more riders, because the platforms and stations have been downsized to save construction costs. The platforms must be lengthened somewhat so they can accommodate trains longer than two cars.

In this city, our transit planning tends to be done on a piecemeal basis - one extension at a time - without a master plan. Muni will not be reformed by merely tinkering with the existing bus system or by trying to limit automobile travel with the hopes of forcing drivers onto a woefully antiquated system. We must envision a citywide rapid transit system, as the voters envisioned for the city 20 years ago, and then flesh out that system and implement it in incremental steps. The Central Subway is one of those steps, and a crucial link in a north-south rapid transit line.

Eight years from now, you should be able to board a T-line train at Fourth and King streets and experience a ride that is quick, comfortable and on-time, to Washington Square. Let us hope that our politicians and bureaucrats have the foresight to develop such a system for us.